Stereocontrol of Reactive Encounters Using Polarized Light

The reaction geometry of selected species can be controlled by using polarized light even in bulk experiments. One reactant A is generated in a photodissociation process. Its spatial distribution is completely described by the anisotropy parameter β. The other molecular reactant B is excited in a specific rovibrational state. Its spatial distribution is given by the j- and branch-dependent alignment parameter A 0 (2). The unnormalized probability of an attack of A on B under an angle γ is then given by the simple expression P(γ) ∝ [1 + 1/5βA 0 (2) P 2(cos γ) P 2(cos δ)], where δ is the angle between the E⃗-vectors of the dissociating and of the exciting laser beams. P 2(x) represents the second Legendre polynom. We have studied the reaction of X + HCN → HX + CN with X = H, Cl. The attacking H atom is generated in the photodissociation of CH3SH at 266 nm, and the chlorine atom is formed in the photolysis of Cl2 at 355 nm. In both cases the β-parameter is close to −1. In order to align the HCN partner reactant, the first and third vibrational overtone of the CH stretch vibration was excited via the R and P branches. The nascent CN(v=0) product molecules were observed by laser induced fluorescence (LIF). The experimental results prove a preferred linear reaction geometry, i.e. an end-on attack of the X atom on the terminating hydrogen atom of the HCN reactant. However, the angle of acceptance is higher for the Cl + HCN reaction than for the H + HCN one.